The invention relates to an electromagnetically force-compensating force-measuring apparatus, comprising                a support coil which is mounted in a permanent magnet arrangement and through which a coil current generated by a controller, depending on a force, flows during operation, and        an integrating analog/digital converter which is designed to convert an electrical signal, which is representative of the coil current and is applied to the measurement signal input thereof, into a digital output signal,wherein the analog/digital converter is connected at the reference voltage input thereof to a reference voltage source which has two reference voltages which have the same magnitude and are oppositely poled relative to one another, and alternately connects a reference voltage switch to an integrator of the analog/digital converter, wherein a ratio of the intervals in which the individual reference voltages are connected to the integrator within a measuring clock cycle is a measure of the presently flowing coil current.        
Digital force-measuring devices operating according to the principle of electromagnetic force compensation, for example digital balances, have long been known. EP 2 253 944 A1 (which corresponds to US 2010/0294573 A1) discloses a digital balance of this type.
According to the measuring principle of electromagnetic force compensation, a support coil connected to a load arm is arranged axially movable in the air gap of a permanent magnet. Current flow through the support coil generates a magnetic field which interacts with the magnetic field of the permanent magnet and leads to a deflection of the coil and of the load arm connected thereto. The position of the load arm is detected by suitable position sensors. The current source for the coil current and the position sensors are components of a control circuit, the control variable of which is the position of the load arm and the manipulated variable of which is the current flow through the support coil. If the load arm is deflected by a force that is to be measured, this deflection is measured by the position sensors and is communicated to a controller which adjusts the current flow through the support coil such that the deflection is counteracted. The current through the support coil is therefore a direct measure of the force acting on the load arm. The coil current or a variable which is representative thereof is digitized in an analog/digital converter (abbreviated: A/D converter) connected downstream. In particular, the principle of the integrating A/D converter is well-known in the art.
A circuit diagram showing the principle of an electronic measurement value detection device 100 of a force-measuring apparatus of this type with a support coil 2, a controller 4 and an A/D converter 10 is shown in FIG. 1.
The heart of the A/D converter 10 is the integrator 12 which comprises an operational amplifier with an inverting input 122, a non-inverting input 123 and an output 124, as well as a capacitor 125 which is connected between the inverting input 122 and the output 124 of the operational amplifier 121. The non-inverting input 123 of the operational amplifier 121 is connected to a reference voltage, particularly to ground. The inverting input 122 is connected via the measuring resistor RM to the measurement voltage input 14 where the measurement voltage UM, which is representative of the coil current IS flowing through the support coil, is applied during operation. In particular, the coil current IS can be converted by a current/voltage converter 6 into the measurement voltage UM. Furthermore, the inverting input 122 is connected via a reference resistor RRef to the reference voltage switch 16 which, depending on the switch setting, electrically connects either the first reference voltage input 18 or the second reference voltage input 20. A reference voltage URef1 or URef2 is applied to each of the reference voltage inputs 18, 20 which typically have inverse polarity relative to one another and can have the same voltage level. The integrator output 126 is connected to the test voltage input 221 of a comparator 22, the reference voltage input 222 of which is connected to a comparator reference voltage, which can be, for example, ground. The comparator 22 outputs a signal or a signal change at its output 223 when the test voltage applied to the test voltage input 221 corresponds to the reference voltage applied to the reference voltage input 222. The comparator output signal is fed back as the switching signal, via a control device 40, to the reference voltage switch 16.
An A/D converter of this type operates as follows: in a first phase of a measuring clock cycle T, the reference voltage switch 16 is switched such that the first reference voltage input 18 is connected. During this phase, the integrator integrates the sum of the measurement current IM, which results from the drop in the measurement voltage UM across the measuring resistor RM, and the reference current IRef1, which results from the drop in the first reference voltage URef1 across the reference resistor RRef. After a time pre-defined by the control device 40, specifically the duration of a first measuring phase t1, which thus represents an integration phase, the reference voltage switch 16 switches over, so that the first reference voltage input 18 is disconnected and the second reference voltage input 20 is connected. Now the integrator integrates the sum of the measurement current IM and the reference current IRef2, which results from the voltage drop in the second reference voltage URef2 across the reference resistor RRef. In this example, the polarities of the measurement voltage UM and the first reference voltage URef1 are opposite and the polarities of the measurement voltage UM and the second reference voltage URef2 are the same. The integrated and deintegrated voltage respectively lie at the integrator output 126 and therefore at the test voltage input 221 of the comparator 22.
This second measuring phase which thus represents a deintegration phase is denoted herein as τ. As soon as the integrator voltage is fully deintegrated, a comparator signal is output which is used by the control device 40 to switch over the reference voltage switch 16 once more and to begin anew measuring clock cycle. Furthermore, the control device 40, which during the preceding measuring clock cycle has measured the durations of the two measuring phases t1=T−τ and τ and, in particular, has calculated the ratio of the duration of the second measuring phase τ to the measuring clock cycle duration T of the preceding measuring clock cycle, i.e. the duty factor δ=τ/T, can output a corresponding numerical value which is a measure of the measurement voltage UM applied during the measuring clock cycle and thus of the coil current IS flowing through the support coil 2.
A disadvantage of the necessary dependence of the coil current IS on the present measurement value is that the power loss arising in the support coil is also measurement value-dependent. The power loss leads to heating of the overall device so that thermal influences (faults) are also measurement value-dependent. This is not tolerable for precision measurements. The above-mentioned application EP 2 253 944 A1 (which corresponds to US 2010/0294573 A1) discloses a possibility for compensating for power losses in the support coil. It is proposed, in particular, to provide the support coil with a double winding, each partial winding being connected to a separate coil current source. The coil current sources are controlled in such a way that the total of their currents generates the electromagnetic forces required to compensate for the weight force, wherein at the same time, the power loss absorbed in the support coil remains constant.
DE 31 49 990 A1 (which corresponds to U.S. Pat. No. 4,450,923) discloses another approach. Herein, in addition to the direct current, an alternating current is also applied to the support coil, in order to generate an overtemperature in the support coil. A direct current and an alternating current proportional to the direct current and the alternating current in the support coil is passed through a strongly temperature-dependent resistor of a voltage divider circuit, in particular a glow wire, the resistance value of which is kept constant by a controller. Based on the proportionality of the currents in the support coil and in the glow wire, a constant power loss is caused in the support coil.